Multiport amplifiers (MPAs) are systems used mainly in the payloads of telecommunications satellites to amplify a plurality of radiofrequency or microwave signals, for example to produce broadband transponders. The operating principle of an MPA consists in using several amplifiers to amplify all the signals concurrently. All the amplifiers, or at least several of them, contribute to the amplification of all the signals; this allows flexible allocation of the power and of the band, while ensuring optimal use of the amplifiers. This operating principle, known per se, is illustrated by FIG. 1 which schematically represents a multiport amplifier exhibiting four input ports PE1-PE4, through which four radiofrequency signals s1-s4 enter, and four output ports PS1-PS4, through which the amplified signals S1-S4 exit. The input ports are linked to the inputs ii1-ii4 of a distributing matrix, or input matrix IBM, which exhibits one and the same number of outputs oi-oi4. This matrix is configured in such a way that the signal si (for example s1, represented by a vector of vertical orientation) present at an input (in this case, i11) is divided over all its outputs with a different phase shift (indicated by the change of orientation of the vectors). Thus, for example, the signal s1 has no phase shift at the output oi1, has a phase shift of 90° at the outputs oi2 and oi3 and has a phase shift of 180° at the output oi4. The same goes, with different phase shifts, for the signals s2-s4 (not represented). This mode of operation corresponds to a “Butler” matrix, known per se.
The signals arising from the outputs oi1-oi4, which are therefore linear combinations of the input signals s1-s4, are amplified by identical power amplifiers PA1-PA4, which may be, for example, of the travelling wave tube (TWTA, standing for “Traveling Wave Tube Amplifier”) or semi-conductor type. The amplified signals are applied to the inputs io1-io4 of a combining matrix, or output matrix OBM, which carries out an operation analogous to that of the distributing matrix; in the case of FIG. 1, OBM is also a Butler matrix.
It may be seen in FIG. 1 that, if the phase shifts introduced by the two matrices are chosen in an opportune manner, the vectors representative of the amplified signal s1 vanish on the outputs oo2-oo4, and are combined on the output oo1 alone. Likewise, the vectors representative of the amplified signal s2 vanish on the outputs oo1, oo3 and oo4, and are combined in a constructive manner on the output oo2 alone, and so on and so forth. Thus, the amplified signal Si, arising from the output port Pi, corresponds to the input signal si amplified, without any contribution from the other input signals sj with j≠i (i,j=1-4).
This manner of operation presupposes ideal input and output matrices and power amplifiers with rigorously identical properties. In reality, this is not the case: the phase shifts introduced by the matrices may be different from the nominal ones, the power of the signals at input may be unequally divided between the outputs, the amplifiers may exhibit different gains and phases, etc. Furthermore, the properties of various elements may drift over time. Because of these discrepancies with respect to an ideal situation, the isolation between the various outputs of the multiport amplifier is not perfect; this signifies that interference terms corresponding to the input signals sj with j·i will be found on each output port PSi.
FIG. 2A shows the spectra of the output signals in the case of a perfectly balanced MPA (the models of the Butler matrices are perfect); the power spectral densities are expressed in dBm and the frequencies (f) in GHz. It may be noted that each output signal Si—which is an amplified version of a corresponding input signal si—exhibits a band which is about 50 MHz in width and a central frequency (frequency of the carrier) which differs, such that the bands of the various signals do not overlap: one then speaks of “disjoint frequency plan”. It may be seen that the isolation is practically perfect (interference level less than −43 dB), since the interferences between signals is invisible to the naked eye. The isolation is defined as the difference (in decibels) of the integrated powers in the band of the reference signal (in the present case, 50 MHz).
FIG. 2B shows the spectra obtained by introducing random imbalances defined by Gaussian distributions of the amplitude error and phase error with σA (standard deviation of the amplitude error)=0.8 dB and σφ (standard deviation of the phase error)=3°; the models of the Butler matrices are “real” (arising from measurements). The interference between signals is clearly visible, and a calculation makes it possible to determine that the interference level is of the order of −22 dB.
The graph of FIG. 2C shows how imbalances in phase (ΔΦ, in degrees “°”) and in amplitude (ΔA, in dB) affect the isolation IS (in dB) between two outputs of a multiport amplifier.
To remedy these imbalances, the amplifier of FIG. 1 comprises weighting elements EP1-EP4, each consisting of an adjustable attenuator (more rarely a preamplifier) and an adjustable phase shifter linked in cascade, associated with respective amplifiers and generally connected upstream of the latter. As only the relative attenuations and phase shifts between the outputs are relevant, one of the weighting elements may optionally be omitted.
By adjusting in an opportune manner the complex weights introduced by these weighting elements it is possible to restore almost ideal isolation—and in any event of greater than 20 dB or more—between the outputs. The calibration operation consisting in adjusting these complex weights can be carried out manually by a technician or, preferably, automatically.
Document WO 2008/135753 describes an automatic method of calibrating a multiport amplifier aimed at maximizing the isolation between the outputs, using a measurement signal or test signal injected into an input. The main drawback of this method is that the injected signal is amplified and, in a telecommunications system, impairs the signal-to-interference ratio C/I.
The article by Mario Caron and Xinping Huang “Estimation and compensation of amplifier gain and phase mismatches in a multiple port amplifier subsystem”, ESA Workshop on Advanced Flexible Telecom Payloads, Nov. 18-20 2008, Noordwijk (Netherlands), discloses an automatic method of calibrating a multiport amplifier not requiring the injection of a measurement signal. This method is based on the study of the probability density functions of the output signals so as to identify, and minimize, the interference between outputs. This is possible only if the type of modulation used is known precisely, thereby limiting the flexibility of the solution.